1. Field of the Invention
The invention relates to a process for preparing a 2-methylenealdehyde by reacting an aldehyde bearing at least two hydrogen atoms on the .alpha.-carbon atom, with formaldehyde in the presence of a catalytic amount of an enamine.
2. Discussion of Prior Art
2-methylenealdehydes (.alpha.-methylenealdehydes, .alpha.-alkylacroleins) can be obtained in various ways. Thus, for example, the reaction of ammonia or a primary or secondary amine generally present in the form of a salt, e.g. as the hydrochloride, with formaldehyde and a compound containing a reactive hydrogen atom leads to the desired methylene compounds (the so-called Mannich reaction). According to the process described in U.S. Pat. No. 2,518,416 a mixture of an aldehyde containing a CH.sub.2 -group in the .alpha.-position to the carbonyl group and formaldehyde is passed through the melt of a salt formed from a primary or secondary amine and a strong acid.
According to the method described in U.S. Pat. No. 2,639,295, the condensation of aliphatic aldehydes with formaldehydes is carried out in the presence of piperidine hydrochloride, morpholine hydrochloride, or an ammonium salt, such as ammonium chloride.
The common feature of the processes mentioned above is that the condensation is carried out in the presence of salts of the amines or ammonia, which are used in stoichiometric amounts or even in excess.
German No. 16 18 528 discloses that .alpha.-methylenealdehydes can be obtained by reacting aldehydes of the general formula RCH.sub.2 CHO with formaldehyde in the presence of catalytic amounts of a primary or secondary amine. However, it is clear from all the examples in which the claimed process is described in more detail, that the amine is always used in the form of its salt and always in amounts which can no longer be considered as catalytic amounts.
The requirement that the condensation of aldehydes containing a CH.sub.2 -group in the .alpha.-position to the carbonyl group with formaldehyde must be carried out in the presence of large amounts of an amine mitigates against economical utilization of the reaction. A further prerequisite for operating with amine salts is the use of steel apparatus in order to avoid damage to the reactors, e.g. damage caused by the occurrence of stress crack corrosion. Finally, the conversion, selectivity and yields in the known processes are unsatisfactory. Limits are, therefore placed on scaling up these processes for industrial production.
It is an object of this invention, therefore, to provide a process for preparing a 2-methylenealdehyde, which process does not have the aforementioned disadvantages and in which the starting substances can be converted in high yields to the desired reaction products, in particular according to a simple reaction procedure.